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Chapter 6

Chapter 6. Interactions Between Cells & the Extracellular Environment. 6-1. Chapter 6 Outline Extracellular Environment Movement Across Plasma Membrane Osmosis Membrane Transport Systems Membrane Potential Cell Signaling. 6-2. Extracellular Environment. 6-3. Extracellular Environment.

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Chapter 6

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  1. Chapter 6 Interactions Between Cells & the Extracellular Environment 6-1

  2. Chapter 6 Outline • Extracellular Environment • Movement Across Plasma Membrane • Osmosis • Membrane Transport Systems • Membrane Potential • Cell Signaling 6-2

  3. Extracellular Environment 6-3

  4. Extracellular Environment • Includes all constituents of body outside cells • 67% of total body H20 is inside cells (=intracellular compartment); 33% is outside cells (=extracellular compartment-ECF) • 20% of ECF is blood plasma • 80% of ECF is interstitial fluid contained in gel-like matrix 6-4

  5. Extracellular Matrix • Is a meshwork of collagen & elastin fibers linked to molecules of gel-like ground substance & to plasma membrane integrins • = glycoprotein adhesion molecules that link intracellular & extracellular compartments • Interstitial fluid resides in hydrated gel of ground substance Fig 6.1 6-5

  6. Movement Across Plasma Membrane 6-6

  7. Transport Across Plasma Membrane • Plasma membrane is selectively permeable--allows only certain kinds of molecules to pass • Many important molecules have transporters & channels • Carrier-mediated transport involves specific protein transporters • Non-carrier mediated transport occurs by diffusion 6-7

  8. Transport Across Plasma Membrane continued • Passive transport moves compounds down concentration gradient; requires no energy • Active transport moves compounds up a concentration gradient; requires energy & transporters 6-8

  9. Diffusion • Is random motion of molecules • Net movement is from region of high to low concentration 6-9

  10. Diffusion continued • Non-polar compounds readily diffuse thru cell membrane • Also some small molecules including C02 & H20 • Diffusion of H20 is called osmosis • Cell membrane is impermeable to charged & most polar compounds • Charged molecules must have an ion channel or transporter to move across membrane 6-10

  11. Diffusion continued • Rate of diffusion depends on: • Magnitude of its concentration gradient • Permeability of membrane to it • Temperature • Surface area of membrane 6-11

  12. Osmosis 6-12

  13. Osmosis • Is net diffusion of H20 across a selectively permeable membrane • H20 diffuses down its concentration gradient • H20 is less concentrated where there are more solutes • Solutes have to be osmotically active • i.e. cannot freely move across membrane Fig 6.5 6-13

  14. Osmosis continued Fig 6.6 • H20 diffuses down its concentration gradient until its concentration is equal on both sides of membrane • Some cells have water channels (aquaporins) to facilitate osmosis 6-14

  15. Osmotic Pressure • Is force that would have to be exerted to stop osmosis • Indicates how strongly H20 wants to diffuse • Is proportional to solute concentration Fig 6.7 6-15

  16. Molarity & Molality • 1 molar solution (1.0M) = 1mole of solute dissolved in 1L of solution • Doesn't specify exact amount of H20 • 1 molal solution (1.0m) = 1 mole of solute dissolved in 1 kg H20 • Osmolality (Osm) is total molality of a solution • E.g. 1.0m of NaCl yields a 2 Osm solution • Because NaCl dissociates into Na+ + Cl- 6-16

  17. Molarity & Molality • Osmolality (Osm) is total molality of a solution • E.g. 1.0m of NaCl yields a 2 Osm solution • Because NaCl dissociates into Na+ & Cl- Fig 6.10 6-17

  18. Tonicity • Is effect of a solution (sln) on osmotic movement of H20 • Isotonic slns have same osmotic pressure • Hypertonic slns have higher osmotic pressure & are osmotically active • Hypotonics have lower osmotic pressure • Isosmotic solutions have same osmolality as plasma • Hypo-osmotic solutions have lower osmotic pressure than plasma • hyperosmotics have higher pressure than plasma 6-18

  19. Effects of tonicity on RBCs Fig 6.11 crenated 6-19

  20. Regulation of Blood Osmolality • Blood osmolality maintained in narrow range around 300m Osm • If dehydrated, osmoreceptors in hypothalamus stimulate: • ADH release • Which causes kidney to conserve H20 • & thirst Fig 6.12 6-20

  21. Membrane Transport Systems 6-21

  22. Carrier-Mediated Transport • Molecules too large & polar to diffuse are transported across membrane by protein carriers 6-22

  23. Carrier-Mediated Transport continued • Protein carriers exhibit: • Specificity for single molecule • Competition among substrates for transport • Saturation when all carriers are occupied • This is called Tm(transport maximum) Fig 6.13 6-23

  24. Facilitated Diffusion • Is passive transport down concentration gradient by carrier proteins Fig 6.15 Fig 6.14 6-24

  25. Active Transport • Is transport of molecules against a concentration gradient • ATP required Fig 6.16 6-25

  26. Na+/K+ Pump • Uses ATP to move 3 Na+ out & 2 K+ in • Against their gradients Fig 6.17 6-26

  27. Secondary Active Transport • Uses energy from “downhill” transport of Na+ to drive “uphill” movement of another molecule • Also called coupled transport • ATP required to maintain Na+ gradient Fig 6.18 6-27

  28. Secondary Active Transport continued • Cotransport (symport) is secondary transport in same direction as Na+ • Countertransport (antiport) moves molecule in opposite direction of Na+ Fig 6.18 6-28

  29. Transport Across Epithelial Membranes • Absorption is transport of digestion products across intestinal epithelium into blood • Reabsorption transports compounds out of urinary filtrate back into blood Fig 6.19 6-29

  30. Transport Across Epithelial Membranes continued • Transcellular transport moves material from 1 side to other of epithelial cells • Paracellular transport moves material through tiny spaces between epithelial cells 6-30

  31. Bulk Transport • Is way cells move large molecules & particles across plasma membrane • Occurs by endocytosis & exocytosis (Ch 3) Fig 6.21 6-31

  32. Membrane Potential 6-32

  33. Membrane Potential Fig 6.22 • Is difference in charge across membrane • Results in part from presence of large anions being trapped inside cell • Diffusable cations such as K+ are attracted into cell by anions • Na+ is not permeable & is pumped out 6-33

  34. Equilibrium Potential • Describes voltage across cell membrane if only 1 ion could diffuse • If membrane permeable only to K+, it would diffuse until reaches its equilibrium potential (Ek) • K+ is attracted inside by trapped anions but also driven out by its [gradient] • At K+ equilibrium, electrical & diffusion forces are = & opposite • Inside of cell has a negative charge of about -90mV Fig 6.23 6-34

  35. Nernst Equation (Ex) • Gives membrane voltage needed to counteract concentration forces acting on an ion • Value of Ex depends on ratio of [ion] inside & outside cell membrane • Ex = 61 log [Xout] z = valence of ion X z [Xin] 6-35

  36. Nernst Equation (Ex) continued • Ex = 61 log [Xout] z [Xin] • For concentrations shown at right: • Calculate EK+ • Calculate ENa+ Fig 6.24 6-36

  37. Nernst Equation (Ex) continued • EK+ = 61 log 5 +1 150 = -90mV • ENa+ = 61 log 145 +1 12 = +60mV Fig 6.24 6-37

  38. Resting Membrane Potential (RMP) • Is membrane voltage of cell in unstimulated state • RMP of most cells is -65 to –85 mV • RMP depends on concentrations of ions inside & out • & on permeability of each ion • Affected most by K+ because it is most permeable 6-38

  39. Resting Membrane Potential (RMP) continued • Some Na+ diffuses in so RMP is less negative than EK+ Fig 6.25 6-39

  40. Role of Na+/K+ Pumps in RMP • Because 3 Na+ are pumped out for every 2 K+ taken in, pump is electrogenic • It adds about -3mV to RMP Fig 6.26 6-40

  41. Cell Signaling 6-41

  42. Cell Signaling • Is how cells communicate with each other • Some use gap junctions thru which signals pass directly from 1 cell to next Fig 7.20 6-42

  43. Cell Signaling continued • In paracrine signaling, cells secrete regulatory molecules that diffuse to nearby target cells • In synaptic signaling, 1 neuron sends messages to another cell via synapses • In endocrine signaling, cells secrete chemical regulators that move thru blood stream to distant target cells • To respond to a chemical signal, a target cell must have a receptor protein for it 6-43

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